By Michael Holden and Estelle Shirbon
LONDON (Reuters) - The British government will need to put forward a new law to trigger formal divorce talks with the European Union if it loses a legal battle over who can start the Brexit process, the Supreme Court heard on Tuesday.
The government is appealing against a ruling last month that it needs parliament's assent to invoke Article 50 of the EU's Lisbon Treaty, the first formal step towards Brexit, as opposed to using an executive power known as the prerogative to do so.
Prime Minister Theresa May has said she intends to trigger Article 50 by the end of March, and the government's fear is that going through parliament could disrupt her timetable and give parliamentarians opportunities to water down its Brexit plans.
Senior government lawyer James Eadie told the Supreme Court that if it upheld the earlier ruling, ministers would have to put forward a new parliamentary bill to trigger Article 50.
"It would require not just parliamentary involvement ... but primary legislation," Eadie told the 11 justices on day two of a four-day appeal hearing.
He also said the new bill could be just a one-line piece of legislation: "If the Supreme Court decides against our arguments here then the solution in legal terms is a one-line act.
"Maybe that would lead to all sorts of parliamentary complication and possible additions and amendments and so on, but that's the solution."
PARLIAMENT V MINISTERS
The government announced on Tuesday it had accepted the Labour Party's call for it to set out its plans for Brexit before formal talks, while asking parliament to respect its timetable.
Britons voted for Brexit by 52 to 48 percent and the government has said this mandated it to begin the divorce process using the prerogative power.
But its position has been challenged in court on the grounds that only parliament can remove rights granted by a parliamentary act in 1972 which paved the way for Britain to join what is now the EU.
"Parliament did not intend that what it had created could be nullified by a minister exercising a prerogative," David Pannick, a lawyer for the lead claimant in the case against the government, told the Supreme Court.
Markets take the view that greater involvement by parliament would reduce the risk of a "hard Brexit". The pound hit a two-month high against the dollar on Tuesday as investors bet the challengers would win in the Supreme Court.
The hearing continues until Thursday and the verdict is expected in January.